Leveraging Conformational and Nitrogen Atom Inversion for Room-Temperature Ferroelectricity

Nitrogen inversion, in which a pyramidalized tricoordinate nitrogen center turns "inside out", is an intriguing phenomenon that has inspired a century of fundamental research but has yet to find practical application. In this work, classical nitrogen inversion is used to template polarization switching in a new class of molecular ferroelectrics, material candidates for next-generation digital information storage systems. We demonstrate that azangulene, a bowl-shaped nitrogen-centered heterotriangulene, when adopting a polar crystal packing motif, exhibits above-room-temperature ferroelectricity that we attribute to whole-molecule inversion. Although the mechanism of classical nitrogen inversion predicts a planar transition-state structure, we isolate a crystallographic polymorph in which the bowl depth of azangulene is flat, suggesting that the planar geometry is a stable and isolable structure on the conformational energy surface. A combination of crystallographic polymorphism and computational investigations unravels the complex interplay between the enthalpic and entropic factors contributing to the unique functionality of this molecule.